1. Technical Field
This invention relates to a process for catalytic cracking of a heavy fraction oil, particularly to a fluid catalytic cracking (FCC) process which comprises cracking a heavy fraction oil to obtain olefins which are light fraction oils such as ethylene, propylene, butene and pentene.
2. Description of the Prior Art
In a usual catalytic cracking technique, petroleum-derived hydrocarbons are catalytically cracked with a catalyst thereby to obtain gasoline as the main product, a small amount of LPG, a cracked gas oil and the like, and coke deposited on the catalyst is then burnt away with air to recycle the regenerated catalyst for reuse.
In recent years, however, there has been a tendency that a fluid catalytic cracking apparatus is utilized not as an apparatus for producing gasoline but as an apparatus for producing light fraction olefins for use as petrochemical materials. Such utilization of an original fluid catalytic cracking apparatus as an olefin producing apparatus is economically advantageous particularly to an oil refinery in which a petroleum refining factory and a petrochemical factory are highly closely combined.
On the other hand, much attention has been paid to environmental problems, and therefore regulation of the contents of olefins and aromatics in gasoline for automobiles, obligation to add oxygen-containing materials (MTBE or the like), or the like has started to be enforced. In consequence, it (an be anticipated that alkylates and MTBE will be increasingly demanded as base materials for high-octane gasoline in place of FCC gasoline and catalytically reformed gasoline. Therefore, it will be necessary to increase the production of propylene and butene which are raw materials for these base materials.
Methods for producing the light fraction olefins by the fluid catalytic cracking of a heavy fraction oil include methods which comprise contacting a raw oil with a catalyst for a shortened time (U.S. Pat. Nos. 4,419,221, 3,074,878 and 5,462,652, and European Patent No. 315,179A), a method which comprises carrying out a cracking reaction at a high temperature (U.S. Pat. No. 4,980,053), and methods which comprise using pentasil type zeolites (U.S. Pat. No. 5,326,465 and Japanese Patent National Publication (Kohyo) No. Hei 7-506389 (506389/95)).
However, these methods have common problems. That is to say, since a raw oil is heated in a reaction zone inlet to gasify it, a catalyst having a higher temperature than a preferable reaction temperature is required to be introduced, so that the raw oil is partially brought into contact with the high-temperature catalyst to bring about cracking; since a cracking reaction is an endothermic reaction, the temperature lowers after the start of the reaction; and since the reaction at the high temperature is severe, a coke is deposited on the catalyst, so that the catalyst rapidly deteriorates. Furthermore, the above methods have another problem that the yield of light fraction olefins lowers owing to overcracking and a hydrogen transfer reaction of the oil which comes out of a reaction zone.